This invention relates to a container for active microwave heating of food products. More particularly, this invention relates to an improved active container system which, surprisingly, is capable of heating or cooking a variety of food products of varying sizes and types. In addition to the pre-cooked and frozen foods that are commonly thawed and reheated in conventional microwave packages, the containers of this invention can be used to thaw and cook frozen foods such as meat. All of these products can be thoroughly and evenly cooked or heated in an energy efficient way, with no significant overcooked, dried or scorched regions.
Microwave heating offers significant advantages in thawing and reheating of food products. Most important, for the ordinary consumer, is the reduced time required to heat many frozen foods. There are substantial drawbacks, however. With conventional packaging, microwave heating is generally uneven, leaving certain areas such as the center of the food product inadequately heated, while regions of the food near the edge of the container tend to be overheated, dried and/or burned.
A variety of designs and approaches have been used to address this problem. Some designs place microwave reflective materials, such as metallic foils, in parts of the container to "shield" parts of the food that tend to be overdone. This reduces the amount of energy reaching these portions of the food, however, which increases cooking times and decreases energy efficiency.
Examples of shielded packages are disclosed in U.S. Pat. Nos. 4,351,997 to Mattison, 3,240,610 to Cease, 3,408,164 to Goltsos and 4,268,738 to Flautt et al, Canadian patent 1,202,088 to Kwis et al. and EPO application 92105572.9 to Saunier. While they reduce overheating of the food around the edges of the package, packages such as these have had limited commercial application. The added cost of the containers has usually overshadowed the potential benefits.
A more recent approach utilizes materials in, or parts of, the package to modify microwave fields therein. This type of packaging, disclosed in U.S. Pat. Nos. 4,656,325 to Keefer, 4,814,568 to Keefer, 4,831,224 to Keefer, 4,866,234 to Keefer, 4,888,459 to Keefer, 4,992,638 to Hewitt et al and 4,990,735 to Lorenson et al, is sometimes referred to as "active" microwave packaging. "Active microwave packaging" has been defined as packaging "that changes the electric (or magnetic) field configuration and thus the heating pattern of the product contained within. Active packaging also includes susceptors or heater boards that are included in a package to brown or crisp a product." Buffler, Charles R., Microwave Cooking and Processing, Engineering Fundamentals for the Food Scientist, Van Nostrand Reinhold, New York, 1993.
Active packages that modify the electrical field make more efficient use of the microwave energy impinging upon them, and provide more even heating of food or other materials in the container. Thus, they make microwave heating practical for many products that could not be heated satisfactorily in other prior art packages. Previous designs of this type, however, have not provided enough control to deal with particularly difficult products, such as relatively large (more than about 300 grams) of uncooked, frozen meat products.